Patent GB-691211-A (GB)
Title: Improvements in and relating to apparatus for controlling the temperature of a fluid medium
IPC: G05D23/275
Filed: 1950-10-20
Abstract: 691,211. Automatic temperature control systems. MARSHALL RICHARDS MACHINE CO., Ltd., and SOMERVILLE, F. B. Oct. 17, 1951 [Oct. 20, 1950], No. 25593/50. Class 38 (iv). [Also in Group XXXVII] The temperature of a fluid medium is automatically controlled by periodically varying the rate of heat input to (or heat evacuation from) the medium, by an amount which is approximately proportional to the difference between the actual temperature of the medium and the desired temperature. The temperature difference is determined by the rate of cooling of a heated wire, preferably by comparing the rate of cooling of two heated wires, one immersed in the fluid medium, and the other placed in a constant ambient temperature. The system is applicable to the control of air temperature in buildings, water or steam temperature in domestic and industrial heating installations, and the temperature of liquid baths and gaseous reaction chambers in industrial processes. The two hot-wire relays are mounted respectively in the controlled chamber or space 1, Fig. 2, and in the standard chamber 1<SP>1</SP>. They comprise fine wires 14, 141 which are simultaneously heated by secondary transformer coils 19, 19<SP>1</SP> when the primary coil 20 is connected across the supply 21 by closure of the switch 22. Expansion of the wires when heated snaps over the switph arms 11, 11<SP>1</SP> from the "cold" contacts 4, 4<SP>1</SP> 'to the "hot" contacts 5, 5<SP>1</SP>, so de-energizing the coils 23, 24 and allowing the supply switch 22 to open as soon as both coils 23, 24 are de-energized. The wires 14, 14<SP>1</SP> now cool by radiation, and if the controlled space 1 is too cold, the arm 11 snaps over to the " cold " contact before the arm 11<SP>1</SP>. A circuit is thus made through a coil 17 which causes a servo-motor to rotate in the direction to increase the rate of heat supply to the space 1. The servo-motor continues to rotate until the contact 11<SP>1</SP> snaps over, whereupon the coil 17 is de-energized and the servomotor stops. Both arms 11, 11<SP>1</SP> being now on the " cold " contacts 4, 4<SP>1</SP>, the relay coils 23, 24 are both connected across the supply, and the switch 22 is again closed to repeat the cycle of heating and cooling. Should the space 1 be too hot, the wire 141 cools more rapidly than the wire 14, and arm 111 snaps over to its cold contact before arm 11. Coil 18 is thus energized to start the servo-motor in the reverse direction to diminish the rate of heat supply. The time interval during which the servomotor operates, and the consequent variation in the rate of heat supply, are proportional to the temperature error. The servo-motor may influence a voltage regulator associated with an electric heater for the controlled medium, or may control the gas supply to a gas heater. If the controlled medium is to be kept cool, the servomotor may control the speed of a fan. The relay for the controlled space 1 has its supporting frame constructed of a material with low temperature-coefficient of expansion; whilst the relay for the standard space 11 may have its frame constructed of material having the same coefficient of expansion as the heated wire. Fig. 1 shows a suitable construction of thermal switch comprising a wire 14 stretched between a fixed point of the frame member 3 and a point 16 on a lever 6 having a virtual pivot where it is flexibly supported by metal tape 8 on a frame pillar 7. The lever 6 is connected by a coil spring 10 to the centre of a snap-action contact arm 11 pivoted at 12 and movable between contacts 4, 5. The wire 14, when heated and extended by a current supply, causes the arm 11 to rest on contact 5. When allowed to cool the wire 14 contracts and swings the lever 6 against the pull of a spring 9. As the lever moves past the position of alignment of the spring 10 with the arm 11, that arm snaps over into engagement with the contact 4.